The applicant's long-term objective is to advance our understanding of the etiologic factors involved in the pathogenesis of age-related ovarian hormone deficiency osteopenia. The overall objective of this pilot study is to characterize a rat model of bone loss due to ovarian hormone deficiency. Specific studies will be carried out to: 1. Determine if ovariectomy causes bone loss in female rats after skeletal maturity is attained. 2. Determine the earliest time when significant bone loss occurs in skeletally mature ovariectomized rats. 3. Determine if the earliest significant bone loss in skeletally mature ovariectomized rats is associated with the known effects of long-term ovarian hormone deficiency on bone dynamics, plasma calcium and circulating hormone levels. The emphasis on skeletal maturity is important since it is desirable that a model to simulate human bone loss should be based on an animal that has achieved skeletal maturity since age-related bone loss in humans starts after the attainment of skeletal maturity. A model of ovarian hormone deficiency bone loss based on the rat will be extremely useful since the rat is a very convenient laboratory animal. Such a model with, therefore, greatly facilitate the exploration of the pathogenesis of bone loss due specifically to ovarian hormone deficiency uncomplicated by other forms of age-related bone loss. In addition, the model will facilitate the evaluation of the efficacy of interventions such as new drugs, dietary manipulation, exercise, etc., that may retard, prevent or reverse bone loss due to ovarian hormone deficiency.